Wipers, certain types of toilet paper, paper napkins and the like are packed in stacks of a fixed height. They are made starting from sheets of absorbent material, for example "tissue" paper, non-woven fabric, etc.
The production starts from a web having a large transversal width, from which sheets are obtained that are folded, stacked and divided into small stacks of a height equal to the final product. Each of these small stacks forms a log of a length equal to said transversal size. The logs, then, are cut off into many short stacks having the final size and packed.
In some cases the sheets obtained from the web are folded separately from one another and then stacked already folded. In other cases the sheets are interfolded, that is, are folded into panels by overlapping at the same time a panel of a previous sheet with a panel of a following sheet. In this way, when pulling a sheet from the stack, at the moment of use, a panel of the following sheet is pulled out, with consequent advantages for certain types of users. Among the possible interfolding ways stacks of L, Z or W interfolded sheets are known having 2, 3 and 4 panels respectively.
For the production of interfolded stacks, machines are known that use one or two webs of paper coming from a reel that are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, the cutting step of the webs into sheets is carried out by cutting rollers that engage with respective blades. In case of L or W interfolding the webs are cut so that they form a sequence of offset sheets coming preferably from two different directions. Therefore, the sheets coming from both directions are supplied alternately to the folding rollers so that each sheet coming from a first direction overlaps a portion of the sheet coming from the second direction, and vice versa.
The sheets coming from both directions, in order to be overlapped in the above described way, adhere to the respective folding rollers by means of a sucking step or by means of a mechanical gripping. Therefore, the downstream portion of each sheet leaves its folding roller at the point of contact between the two rollers, then adhering to the other folding roller, to which the upstream portion of the previous sheet adhered.
The interfolded stacking step is carried out by fixed shrouds or by folding arms that have an oscillating motion about a pivot and that in turn push away from the respective roller the upstream portion of each sheet joined to the overlapped downstream portion of the following sheet.
In case of Z interfolding there is the only difference that two consecutive sheets overlap each other just after the cutting step and the sequence of overlapping and offset sheets come to the folding rollers from only one direction.
In the machine in which the folding rollers comprise mechanical grippers there is the drawback that such system is very expensive and complex to use.
Instead, in the machine in which the sheets adhere to the folding rollers through sucking means, the downstream portions of the sheets are held owing to their permeability to air. In fact, they are held in turn by one or by the other folding roller through the forces generated by air owing to the friction pressure loss throughout the paper.
Therefore, the method of holding the sheets by sucking, advantageous because less expensive of the gripping means, cannot be used when the material to interfold is not or not enough permeable to air. This may occur when the web is thicker than usual or because the web has special physical features.
Consequently, the need is felt of a method and of an apparatus for interfolding sheet material not or not enough permeable to air that is easier to control and less expensive than the gripping means.
It is therefore object of the present invention to provide a new interfolding method of sheet material not or not enough permeable to air.
It is another object of the present invention to provide an apparatus for carrying out such a method.